Sedimentology, coal chemistry and petrography of the Cretaceous Morley coal measures and the Eocene Beaumont coal measures, Ohai Coalfield, South Island, New Zealand.

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Thesis Discipline

Doctor of Philosopy

Geology

Degree Grantor

University of Canterbury

Degree Level

Doctoral

Degree Name

Doctor of Philosophy

Several kinds of data from the Cretaceous Morley and Eocene Beaumont Coal
Measures at Ohai Coalfield were integrated in order to develop models for sedimentary
elastic deposition and mire formation. These data comprise lithostratigraphic relationships at
outcrop and basin scale and information on coal chemistry, petrography and palynology.
Results indicate that although deposition of both Morley and Beaumont Coal Measures were
tectonically controlled by the development of sedimentary sub-basins, Cretaceous mires and
sedimentary regimes differ from those of the Eocene.
Although the data available at Ohai Coalfield are insufficient for interpretation of fluvial
channel planform, other characteristics of the sedimentary environments can be deduced.
Accumulation of the Morley Coal Measures occurred in two types of non-contemporaneous
environment, 'S'-environments, in which widespread sand was deposited by fluvial
channels and few mires developed, and 'C'-environments in which only fine-grained elastic
deposition occurred and mires were extensive and persistent. Three environments, which
were sometimes contemporaneous, have been identified in the Beaumont Coal Measures. In
Beaumont 'S'-environments, sandy elastic sediment was deposited widely by fluvial
channels whereas in the 'C'- and 'C-S'-environments, mires developed and sedimentation in
shallow lakes and streams was widespread. In the 'C'-environments channels carried mud
with little sand but in the 'C-S'-environments sand deposition was more common.
Morley mires, which were larger and longer-lived than Beaumont mires, were rarely
flooded and may have been domed. In contrast, Beaumont mires were frequently flooded
and probably not domed. Most Morley mires developed in environments with widespread
mires drained by low energy streams. In contrast high energy fluvial activity was more
common in Beaumont mire-forming environments; Beaumont mires are inferred to have
frequently developed on lake margins. Palynological evidence indicates that the Morley
floral assemblage was dominated by gymnosperms whereas both angiosperms and
gymnosperms formed significant proportions of the Beaumont flora.
The information available on Morley coal allows development of a model for peat
accumulation. Peat accumulation was influenced by a number of interdependent parameters
including water table level, nutrient supply and acidity. In response to environmental
conditions two different peat types formed. At the base, top and margins of mires peat was
generally woodier, less degraded and less oxidised. In contrast, peat in the mire centres
suffered both more non-oxidative degradation as well as oxidation and contained less woody
material. As mires developed, the initially diverse gymnosperm flora became dominated by
the podocarp Phyllocladidites mawsonii however this change in vegetation did not affect
the character of the peat.